RAS P21 FARNESYLATION IN ULTRAVIOLET-B RADIATION - INDUCED-TUMORS IN THE SKIN OF SKH-1 HAIRLESS MICE

Cutaneous exposure to solar ultraviolet B (UVB) radiation is well reco gnized as the major cause of skin cancer in humans; however, ther prec ise molecular mechanisms whereby UVB mediates carcinogenesis remains u nclear. The involvement of activated ras oncogenes has been demonstrat ed extensively in both animal and human skin cancers. Activated ras on cogenes encode mutated ras p21 that exist in the guanosine triphosphat e-bound active state and, following localization to the inner side of the plasma membrane, cause cellular transformation. This membrane asso ciation requires three post-translational modifications occurring at t he C-terminus of the ras p21. The farnesylation of p21 by a cytosolic enzyme known as farnesyltransferase (FTase) is the critical step that triggers biologic functions of the ras p21. In this study, FTase activ ity was found to be substantially higher (approximately threefold) in UVB radiation-induced tumors in SKH-1 hairless mice compared to epider mis from controls. Western blot analysis showed significantly higher l evels of Ha-ras p21 in both cytosolic and membrane fractions prepared from tumors compared to epidermis. Pan ras antibody against mutated p2 1 at codon 12 showed very strong reactivity for ras val-12p21 in tumor s but not in normal epidermis, suggesting a gly to val substitution at 12th position in ras p21 in UVB-induced tumors. Our data indicate tha t enhanced FTase activity and the processing of overexpressed p21 in U VB-induced tumors are correlated, and predict the role of point mutati on at the 12th codon of the ras oncogene during photocarcinogenesis in mice.